This research is designed to investigate the influence of the recipient microenvironment on human haploidentical donor stem cells which mature to become functioning T lymphocytes. While evidence exists in mice and Xenopus for thymic "education" of such cells to recognize recipient major histocompatibility complex (MHC) antigens as self, it is not yet known whether this is true for man. The overall goal of the proposed research is to study the ontogeny, competence, tolerance and self-recognition specificities of T cells from infants with severe combined immunodeficiency who have received haploidentical (half- matched) stem cells isolated by soy lectin agglutination and sheep erythrocyte rosetting from opposite sex parental donors. The aims are: 1) to define the sequential appearance post- transplantation of blood cells with immature and mature lymphocyte surface antigens, using a panel of monoclonal antibodies and multiparameter cytofluorography, and of cells with evidence of rearranged T cell receptor (Ti) genes, using cDNA probes to human Ti alpha and beta chain genes and in situ hybridization, 2) to correlate the appearance of these with a range of in vivo and in vitro immune functions, 3) to establish the genetic origin of the various types of immune cells (T, B, NK and monocytes) by karyotyping, HLA typing or DNA sequence polymorphism analysis, 4) to define the self-recognition specificities of the genetically donor T cells in the patient through mixed leukocyte culture experiments, and 5) to define the major histocompatibility complex restriction pattern of antigen recognition by tetanus toxoid-specific, genetically donor helper T cell clones derived from the blood of such chimeras using Epstein- Barr virus transformed B cell lines from the relevant genetic sources as antigen-presenting cells. It is important to know whether the efficacy of such transplants is due to the fact that the genetically donor T cells share a common HLA haplotype with host B and antigen-presenting cells or is due to a change in the donor T cells as they mature in the recipient which enables them to see recipient HLA antigens as self. If the latter, such transplants could conceivably be successful from unrelated donors.